Conventional crossflow cooling towers 10, as shown in FIG. 1, are presently in widespread use and generally comprise a relatively narrow upright fill section 11 with initially hot water being fed from an overhead supply source 12 and the air being drawn horizontally therethrough from air inlets at the side of the tower 10. As the water descends in an even distribution along the upright fill section 11, the cooling crossflow air currents (arrows 13) intersect the descending water in a heat exchanging relation. Subsequently, the cooled water is collected in a water basin 14 below, while the hot, moist air is discharged into the atmosphere.
In a crossflow cooling tower, since there is no necessity for the air to make radical changes of direction into the fill and the air inlet is spaced along the entire height of the fill, the overall air pressure losses in the fill are usually less than those of a conventional counterflow tower. Hence, air can be more easily passed through the tower, such as by powered fan 15.
As illustrated in FIG. 1, crossflow fill sections 11, 11' and the framing to accommodate these sections are normally standardized designs dimensioned in a cross-sectional parallelogram configuration which primarily depend upon the size of the cooling tower 10. This cross-sectional configuration basically conforms to the drift angle or trajectory of the water as it is released from the overhead water supply source 12 and is influenced by air flowing through the gas inlet opening 16 horizontally at right angles to the falling water. Thus, this drift angle is primarily a function of the velocity of the entering air as it impinges on the falling water.
Conventional crossflow cooling towers, and some counter flow cooling towers, generally employ various varieties of splash-type fill sections 17 consisting of elongated bars of a specific configuration for dispersing the descending released water. More recently, film-type fill sections 18 have been developed which have proven substantially more efficient than splash fill sections. These typically corrugated film fills generally consist of a series of thin, opposed sheets formed of synthetic resin materials in which the water passes along the sheets in a "film". Although the film fill sections 18 may be as much as five times as efficient as splash fill sections, their substantial cost differential and higher resistance (i.e., higher static pressure) to air flow has prevented total supplanting of the latter in at least one design as shown in U.S. Pat. No. 5,023,022. In part for these reasons, the most efficient fill section designs normally incorporate splash fill 17 inboard of the film fill 18 to accommodate additional water flow.
Unfortunately, a crossflow cooling tower is inherently less efficient with respect to heat transfer than a counterflow tower based on a unit of fill. Another disadvantage of the crossflow cooling tower is that the water is loaded onto the top of the relatively thin crossflow fill. There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the tower. When this maximum water load is exceeded in a crossflow tower of the film fill type, the water will not cling to the fill, leading to relatively poor heat transfer between the air and water. For example, it is not practical to consider loading film fill beyond a bulk inlet liquid velocity of about 1 meter per minute, and usually the practical limit can be 20% to 25% lower than this. On the other hand, high water loadings do expose the maximum amount of water to the high capability film surfaces, and at the same time tend to flush foulants off these film surfaces.
This water loading problem cannot be effectively overcome by horizontally widening the fill section 11 in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill.
Typically, the uppermost coolant air, flowing horizontally across the upper portion of the fill section 11, is exposed to the hottest inlet water during the entire traverse through the fill section. This continual contact of the coolant air with this hot inlet water causes near thermal saturation of the air.
As a result, especially with the high heat transfer capability typical of film fill, the temperature of the heated air in this region of the tower 10 approaches that of the hot inlet water which it contacts as it passes into the inboard portions of the fill section 11. It is often found that the temperature of the air can be within 1.degree. C. of the water it has cooled in these upper inboard filled portions of a tower 10. Accordingly, the ability of the air to absorb more heat when the air temperature is near the water temperature is significantly reduced.
Hence, the mere widening of the splash fill sections 17 inboard of the film fill section 18 to accommodate more water flow results in very little additional cooling performance due to the typically low heat and mass transfer capability of splash fills in general. Various fill designs have been proposed which account for this lack of performance in the upper inboard portion of the fill section 11, particularly in those towers utilizing splash fill. Usually, high performance fill is positioned in the upper outer corner of the tower where the water is at its hottest and contacts the coolest air. This condition puts crossflow cooling at its optimum capability. To maximize the amount of cooling that can occur in the upper outer corner, the water loading can be increased there so that as much of the water as practical can be exposed to this ideal condition. Typical of these patent designs include U.S. Pat. Nos.: 5,427,718; 5,283,012; 5,023,022; 4,826,636; and 4,460,521.
Another major disadvantage in widening the fill section 11 is that the resistance to air flow for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems. Film fill, particularly at high water loading, presents a high resistance (high static pressure) to air flow. As a result, it is not advantageous nor economical to utilize film fill 18 of excessive air travel thickness. Hence, this is another reason for incorporating the less resistant, more economical, splash fill 17 inboard of the film fill 18 to accommodate additional water flow.
The increased flow resistance, experienced by most cooling towers incorporating film fill sections, is typically compensated by increasing the fan power or capacity which draws air from the exhaust plenum chamber. This increase in capacity is usually accomplished by increasing the blade diameter. However, for air flow efficiency reasons and for the proper fan performance, to accommodate a larger diameter blade 19 of fan 15, the overall tower structure must be increased rather than merely increasing the diameter of the exhaust stack 20 housing the fan 15. Similar air flow efficiency considerations apply to natural draft cooling towers where the fan is replaced by a high stack.
As a general rule, the tip of the stack 20 should not substantially extend horizontally outboard past the upper opening 21 of the exhaust plenum chamber 22 (FIG. 1). Such an arrangement adversely affects the fan or stack efficiency since the exhausted air from exhaust plenum chamber 22 cannot be directly supplied, in the vertical direction, to the outer portions of the fan 15 or stack 20 which extend over the fill section 11. The air exhausted by these outer portions of the fan would have to travel in a direction vertically through upper opening 21 and then diagonally over the fill section 11 before reaching the outer fan portions. Moreover, this can further decrease the life of the fan 15 since the blades tend to unload as they pass across the area of the disturbed air flow above the fill. This sporadic unloading results in excessive blade vibration.
Accordingly, fan blade 19 and stack 20 are preferably diametrically similar to and are positioned vertically above the upper opening 21, as shown in FIG. 1. The problem with this design limitation is that the maximum fan dimensions are largely dictated by the height and slope of the fill section gas outlet portion, as well as the separation between the opposing fill sections 11, 11'. For instance, the improved cooling capability of film fill, as opposed to splash fill, enables lower cooling tower heights. In turn, the lower tower height requires volumetrically less air traveling through the tower than a standard, taller, all splash fill tower. Hence, it is reasonable to deduce that a smaller capacity fan, and therefore a small upper opening, would be appropriate.
This is not the case, however. In a smaller diameter fan, the working area and the corresponding air moving capability of the fan is reduced by the square of the diameter. Thus, the normal air requirement is reduced in direct proportion to the fill section height, while the working exhaust area into the fan or stack is reduced by the square of the reduction in the fill section height. Not only is the air flow that can be drawn into and exhausted from a tower cell limited, but also the length of the cell that the smaller fan can service must be reduced.
Another problem associated with a crossflow cooling tower having too small an upper opening is that as the air flows horizontally through the fill section 11 and exits into the exhaust plenum chamber 22, it must turn upwards 90.degree. to exit through exhaust stack 20. The incoming air must then compete for space as it cumulatively enters the exhaust plenum chamber. Hence, directional transitions of the air flowing into the plenum chamber which are not smooth may result in problematic internal pressure losses which result from high velocity air currents turning upward and competing for limited plenum volume.
The above-mentioned flow problems can be remedied through widening of the upper opening 21 to accommodate a larger capacity fan. This is usually accomplished by spacing the two opposing fill sections 11, 11' further apart or increasing the height of fill sections, or a combination thereof. The former structural arrangement results in a wider tower, while the latter causes an increase the overall height of the tower.